\begin{table}[h]

\caption{\label{tab:t_a:panelmortality}Number of respondents per group and wave. Column `%' indicates 
        the percentage of respondents retained in each wave (relative to the first wave).}
\centering
\begin{tabular}[t]{lrrrrrr}
\toprule
\multicolumn{1}{c}{ } & \multicolumn{2}{c}{Wave 1} & \multicolumn{2}{c}{Wave 2} & \multicolumn{2}{c}{Wave 3} \\
\cmidrule(l{3pt}r{3pt}){2-3} \cmidrule(l{3pt}r{3pt}){4-5} \cmidrule(l{3pt}r{3pt}){6-7}
Group & N & \% & N & \% & N & \%\\
\midrule
1 & 1077 & 100 & 581 & 53.95 & 436 & 40.48\\
2 & 1939 & 100 & 916 & 47.24 & 752 & 38.78\\
3 & 881 & 100 & 403 & 45.74 & 341 & 38.71\\
\bottomrule
\end{tabular}
\end{table}
